Multi-type-tree framework for transform in video coding

ABSTRACT

Embodiments include methods and apparatuses for decoding video data including receiving an encoded video bitstream that forms a representation of a coded picture of the video data and determining a partitioning of the coded picture of the video data into a plurality of coded unit. The partitioning may be according to a first tree structure and the plurality of coded units including a leaf node in the first tree structure. A method may further include determining that a residual block of the leaf node is recursively split into a plurality transform units according to a second tree structure.

CLAIM OF PRIORITY UNDER 35 U.S.C. § 119

The present Application for Patent claims priority to Provisional Application No. 62/651,689, filed Apr. 2, 2018, and assigned to the assignee hereof and hereby expressly incorporated by reference herein.

TECHNICAL FIELD

This disclosure relates to video encoding and video decoding.

BACKGROUND

Digital video capabilities can be incorporated into a wide range of devices, including digital televisions, digital direct broadcast systems, wireless broadcast systems, personal digital assistants (PDAs), laptop or desktop computers, tablet computers, e-book readers, digital cameras, digital recording devices, digital media players, video gaming devices, video game consoles, cellular or satellite radio telephones, so-called “smart phones,” video teleconferencing devices, video streaming devices, and the like. Digital video devices implement video coding techniques, such as those described in the standards defined by MPEG-2, MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4, Part 10, Advanced Video Coding (AVC), the High Efficiency Video Coding (HEVC) standard, and extensions of such standards. The video devices may transmit, receive, encode, decode, and/or store digital video information more efficiently by implementing such video coding techniques.

Video coding techniques include spatial (intra-picture) prediction and/or temporal (inter-picture) prediction to reduce or remove redundancy inherent in video sequences. For block-based video coding, a video slice (e.g., a video picture/frame or a portion of a video picture) may be partitioned into video blocks, which may also be referred to as treeblocks, coding units (CUs) and/or coding nodes. Pictures may be referred to as frames. Reference pictures may be referred to as reference frames.

Spatial or temporal prediction results in a predictive block for a block to be coded. Residual data represents pixel differences between the original block to be coded and the predictive block. For further compression, the residual data may be transformed from the pixel domain to a transform domain, resulting in residual transform coefficients, which then may be quantized. Entropy coding may be applied to achieve even more compression.

SUMMARY

This disclosure describes techniques for partitioning blocks of video data using a multiple tree-based frameworks including recursively splitting a transform block or unit into a tree structure that may differ from that of the corresponding coding unit.

One embodiment includes a method of decoding video data. The method includes receiving an encoded video bitstream that forms a representation of a coded picture of the video data. The method further includes determining a partitioning of the coded picture of the video data into a plurality of coded units. The partitioning is according to a first tree structure. The plurality of coded units include a leaf node of the first tree structure. The method further includes determining that a residual block of the leaf node is recursively split into a plurality of transform units according to a second tree structure. The method further includes reconstructing the coded picture based on the determined first and second tree structures.

Another embodiment includes a method of encoding video data. The method includes obtaining a picture for encoding into an encoded video bitstream and determining a partitioning of the picture of the video data into a plurality of coded units. The partitioning is according to a first tree structure he plurality of coded units includes a leaf node of the first tree structure. The method further includes determining that a residual block of the leaf node is recursively split into a plurality of transform units according to a second tree structure. The method further includes encoding, into the encoded video bitstream, the picture based on the determined first and second tree structures.

Another embodiment includes an apparatus for decoding video data. The apparatus includes a memory configured to store a coded picture of the video data. The apparatus further includes a video processor configured to receive an encoded video bitstream that forms a representation of the coded picture of the video data and determine a partitioning of the coded picture of the video data into a plurality of coded units. The partitioning is according to a first tree structure. The plurality of coded units includes a leaf node of the first tree structure. The processor is further configured to determine that a residual block of the leaf node is recursively split into a plurality of transform units according to a second tree structure and to reconstruct the coded picture based on the determined first and second tree structures.

Another embodiment includes an apparatus for encoding video data. The apparatus includes a memory configured to store a coded picture of the video data. The apparatus further includes a video processor configured to obtain a picture for encoding into an encoded video bitstream and to determine a partitioning of the picture of the video data into a plurality of coded units. The partitioning is according to a first tree structure he plurality of coded units includes a leaf node of the first tree structure. The processor is further configured to further determine that a residual block of the leaf node is recursively split into a plurality of transform units according to a second tree structure. The processor is further configured to encode, into the encoded video bitstream, the picture based on the determined first and second tree structures.

Another embodiment includes an apparatus for decoding video data. The apparatus includes a means for storing a coded picture of the video data. The apparatus further includes a means for obtaining an encoded video bitstream that forms a representation of the coded picture of the video data and means for determining a partitioning of the coded picture of the video data into a plurality of coded units. The partitioning is according to a first tree structure. The plurality of coded units includes a leaf node of the first tree structure. The apparatus further includes means for determining that a residual block of the leaf node is recursively split into a plurality of transform units according to a second tree structure and means for reconstructing the coded picture based on the determined first and second tree structures.

Another embodiment includes an apparatus for encoding video data. The apparatus includes means for storing a coded picture of the video data. The apparatus further includes means for obtaining a picture for encoding into an encoded video bitstream and means for determining a partitioning of the picture of the video data into a plurality of coded units. The partitioning is according to a first tree structure he plurality of coded units includes a leaf node of the first tree structure. The apparatus further includes means for determining that a residual block of the leaf node is recursively split into a plurality of transform units according to a second tree structure. The apparatus further includes means for encoding, into the encoded video bitstream, the picture based on the determined first and second tree structures.

The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description, drawings, and claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example video encoding and decoding system configured to implement techniques of the disclosure.

FIG. 2 is a conceptual diagram illustrating coding unit (CU) structure in High Efficiency Video Coding (HEVC).

FIG. 3 is a conceptual diagram illustrating example partition types for an inter prediction mode.

FIG. 4A is a conceptual diagram illustrating an example of block partitioning using a quad-tree-binary-tree (QTBT) structure.

FIG. 4B is a conceptual diagram illustrating an example tree structure corresponding to the block partitioning using the QTBT structure of FIG. 4A.

FIG. 5A is a conceptual diagram illustrating example horizontal triple-tree partition types.

FIG. 5B is a conceptual diagram illustrating example horizontal triple-tree partition types.

FIG. 5C is a conceptual diagram illustrating example asymmetric binary tree partition types.

FIG. 6A is a conceptual diagram illustrating quad-tree partitioning.

FIG. 6B is a conceptual diagram illustrating vertical binary-tree partitioning.

FIG. 6C is a conceptual diagram illustrating horizontal binary-tree partitioning.

FIG. 6D is a conceptual diagram illustrating vertical center-side tree partitioning.

FIG. 6E is a conceptual diagram illustrating horizontal center-side tree partitioning.

FIG. 7 is a conceptual diagram illustrating an example of coding tree unit (CTU) partitioning according to the techniques of this disclosure.

FIG. 8 is a block diagram illustrating an example of a video encoder.

FIG. 9 is a block diagram illustrating an example of a video decoder.

FIG. 10A is a flowchart illustrating an example operation of a video encoder, in accordance with a technique of this disclosure.

FIG. 10B is a flowchart illustrating an example operation of a video decoder, in accordance with a technique of this disclosure.

FIG. 11 is a flowchart illustrating an example operation of a video encoder, in accordance with another example technique of this disclosure.

FIG. 12 is a flowchart illustrating an example operation of a video decoder, in accordance with another example technique of this disclosure.

DETAILED DESCRIPTION

This disclosure is related to the partitioning and/or organization of blocks of video data (e.g., coding units) in block-based video coding. The techniques of this disclosure may be applied in video coding standards. In various examples described below, the techniques of this disclosure include partitioning blocks of video data using three or more different partitioning structures. In some examples, three or more different partition structures may be used at each depth of a coding tree structure. Such partitioning techniques may be referred to as multi-type-tree (MTT) partitioning. By using MTT partitioning, video data may be more flexibly partitioned, thus allowing for greater coding efficiency.

FIG. 1 is a block diagram illustrating an example video encoding and decoding system 10 that may utilize techniques of this disclosure for partitioning blocks of video data, signaling and parsing partition types, and applying transforms and further transform partitions. As shown in FIG. 1, system 10 includes a source device 12 that provides encoded video data to be decoded at a later time by a destination device 14. In particular, source device 12 provides the video data to destination device 14 via a computer-readable medium 16. Source device 12 and destination device 14 may comprise any of a wide range of devices, including desktop computers, notebook (i.e., laptop) computers, tablet computers, set-top boxes, telephone handsets such as so-called “smart” phones, tablet computers, televisions, cameras, display devices, digital media players, video gaming consoles, video streaming device, or the like. In some cases, source device 12 and destination device 14 may be equipped for wireless communication. Thus, source device 12 and destination device 14 may be wireless communication devices. Source device 12 is an example video encoding device (i.e., a device for encoding video data). Destination device 14 is an example video decoding device (e.g., a device or apparatus for decoding video data).

In the example of FIG. 1, source device 12 includes a video source 18, a storage media 20 configured to store video data, a video encoder 22, and an output interface 24. Destination device 14 includes an input interface 26, a storage medium 28 configured to store encoded video data, a video decoder 30, and display device 32. In other examples, source device 12 and destination device 14 include other components or arrangements. For example, source device 12 may receive video data from an external video source, such as an external camera. Likewise, destination device 14 may interface with an external display device, rather than including an integrated display device.

The illustrated system 10 of FIG. 1 is merely one example. Techniques for processing video data may be performed by any digital video encoding and/or decoding device or apparatus. Although generally the techniques of this disclosure are performed by a video encoding device and a video decoding device, the techniques may also be performed by a combined video encoder/decoder, typically referred to as a “CODEC.” Source device 12 and destination device 14 are merely examples of such coding devices in which source device 12 generates encoded video data for transmission to destination device 14. In some examples, source device 12 and destination device 14 operate in a substantially symmetrical manner such that each of source device 12 and destination device 14 include video encoding and decoding components. Hence, system 10 may support one-way or two-way video transmission between source device 12 and destination device 14, e.g., for video streaming, video playback, video broadcasting, or video telephony.

Video source 18 of source device 12 may include a video capture device, such as a video camera, a video archive containing previously captured video, and/or a video feed interface to receive video data from a video content provider. As a further alternative, video source 18 may generate computer graphics-based data as the source video, or a combination of live video, archived video, and computer-generated video. Source device 12 may comprise one or more data storage media (e.g., storage media 20) configured to store the video data. The techniques described in this disclosure may be applicable to video coding in general, and may be applied to wireless and/or wired applications. In each case, the captured, pre-captured, or computer-generated video may be encoded by video encoder 22. Output interface 24 may output the encoded video information to computer-readable medium 16.

Destination device 14 may receive the encoded video data to be decoded via computer-readable medium 16. Computer-readable medium 16 may comprise any type of medium or device capable of moving the encoded video data from source device 12 to destination device 14. In some examples, computer-readable medium 16 comprises a communication medium to enable source device 12 to transmit encoded video data directly to destination device 14 in real-time. The encoded video data may be modulated according to a communication standard, such as a wireless communication protocol, and transmitted to destination device 14. The communication medium may comprise any wireless or wired communication medium, such as a radio frequency (RF) spectrum or one or more physical transmission lines. The communication medium may form part of a packet-based network, such as a local area network, a wide-area network, or a global network such as the Internet. The communication medium may include routers, switches, base stations, or any other equipment that may be useful to facilitate communication from source device 12 to destination device 14. Destination device 14 may comprise one or more data storage media configured to store encoded video data and decoded video data.

In some examples, encoded data (e.g., encoded video data) may be output from output interface 24 to a storage device. Similarly, encoded data may be accessed from the storage device by input interface 26. The storage device may include any of a variety of distributed or locally accessed data storage media such as a hard drive, Blu-ray discs, DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or any other suitable digital storage media for storing encoded video data. In a further example, the storage device may correspond to a file server or another intermediate storage device that may store the encoded video generated by source device 12. Destination device 14 may access stored video data from the storage device via streaming or download. The file server may be any type of server capable of storing encoded video data and transmitting that encoded video data to the destination device 14. Example file servers include a web server (e.g., for a website), an FTP server, network attached storage (NAS) devices, or a local disk drive. Destination device 14 may access the encoded video data through any standard data connection, including an Internet connection. This may include a wireless channel (e.g., a Wi-Fi connection), a wired connection (e.g., DSL, cable modem, etc.), or a combination of both that is suitable for accessing encoded video data stored on a file server. The transmission of encoded video data from the storage device may be a streaming transmission, a download transmission, or a combination thereof.

The techniques of this disclosure may be applied to video coding in support of any of a variety of multimedia applications, such as over-the-air television broadcasts, cable television transmissions, satellite television transmissions, Internet streaming video transmissions, such as dynamic adaptive streaming over HTTP (DASH), digital video that is encoded onto a data storage medium, decoding of digital video stored on a data storage medium, or other applications. In some examples, system 10 may be configured to support one-way or two-way video transmission to support applications such as video streaming, video playback, video broadcasting, and/or video telephony.

Computer-readable medium 16 may include transient media, such as a wireless broadcast or wired network transmission, or storage media (that is, non-transitory storage media), such as a hard disk, flash drive, compact disc, digital video disc, Blu-ray disc, or other computer-readable media. In some examples, a network server (not shown) may receive encoded video data from source device 12 and provide the encoded video data to destination device 14, e.g., via network transmission. Similarly, a computing device of a medium production facility, such as a disc stamping facility, may receive encoded video data from source device 12 and produce a disc containing the encoded video data. Therefore, computer-readable medium 16 may be understood to include one or more computer-readable media of various forms, in various examples.

Input interface 26 of destination device 14 receives information from computer-readable medium 16. The information of computer-readable medium 16 may include syntax information defined by video encoder 22 of video encoder 22, which is also used by video decoder 30, that includes syntax elements that describe characteristics and/or processing of blocks and other coded units, e.g., groups of pictures (GOPs). Storage media 28 may store encoded video data received by input interface 26. Display device 32 displays the decoded video data to a user. Display device 32 may comprise any of a variety of display devices such as a cathode ray tube (CRT), a liquid crystal display (LCD), a plasma display, an organic light emitting diode (OLED) display, or another type of display device.

Video encoder 22 and video decoder 30 each may be implemented as any of a variety of suitable encoder or decoder circuitry, such as one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), discrete logic, software, hardware, firmware or any combinations thereof. When the techniques are implemented partially in software, a device may store instructions for the software in a suitable, non-transitory computer-readable medium and may execute the instructions in hardware using one or more processors to perform the techniques of this disclosure. Each of video encoder 22 and video decoder 30 may be included in one or more encoders or decoders, either of which may be integrated as part of a combined encoder/decoder (CODEC) in a respective device. Video encoder 22 and video decoder 30 may include ALUs, EFUs, digital circuits, analog circuits, and/or programmable cores formed from programmable circuits. In examples where the operations of video encoder 22 and/or video decoder 30 are performed by software executing on the programmable circuits, on-chip or off-chip memory may store instructions (e.g., object code) of the software that video encoder 22 or video decoder 30 receives and executes.

In some examples, video encoder 22 and video decoder 30 may operate according to a video coding standard, such as ITU-T H.265, also referred to as High Efficiency Video Coding (HEVC) or extensions thereto, such as the multi-view and/or scalable video coding extensions. Alternatively, video encoder 22 and video decoder 30 may operate according to other proprietary or industry standards, such as the Joint Exploration Test Model (JEM) or ITU-T H.266, also referred to as Versatile Video Coding (VVC). A recent draft of the VVC standard is described in Bross, et al. “Versatile Video Coding (Draft 3),” Joint Video Experts Team (JVET) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 12^(th) Meeting: Macao, CN, 3-12 Oct. 2018, JVET-L1001-v9 (hereinafter “VVC Draft 3). The techniques of this disclosure, however, are not limited to any particular coding standard. Rather, examples are described relative to the above examples for purposes of illustration.

In HEVC and other video coding specifications, a video sequence typically includes a series of pictures. Pictures may also be referred to as “frames.” A picture may include three sample arrays, denoted S_(L), S_(Cb), and S_(Cr). S_(L) is a two-dimensional array (i.e., a block) of luma samples. S_(Cb) is a two-dimensional array of Cb chrominance samples. S_(Cr) is a two-dimensional array of Cr chrominance samples. Chrominance samples may also be referred to herein as “chroma” samples. In other instances, a picture may be monochrome and may only include an array of luma samples.

Furthermore, in HEVC and other video coding specifications, to generate an encoded representation of a picture, video encoder 22 may generate a set of coding tree units (CTUs). Each of the CTUs may comprise a coding tree block of luma samples, two corresponding coding tree blocks of chroma samples, and syntax structures used to code the samples of the coding tree blocks. In monochrome pictures or pictures having three separate color planes, a CTU may comprise a single coding tree block and syntax structures used to code the samples of the coding tree block. A coding tree block may be an N×N block of samples. A CTU may also be referred to as a “tree block” or a “largest coding unit” (LCU). The CTUs of HEVC may be broadly analogous to the macroblocks of other standards, such as H.264/AVC. However, a CTU is not necessarily limited to a particular size and may include one or more coding units (CUs). A slice may include an integer number of CTUs ordered consecutively in a raster scan order.

If operating according to HEVC, to generate a coded CTU, video encoder 22 may recursively perform quad-tree partitioning on the coding tree blocks of a CTU to divide the coding tree blocks into coding blocks, hence the name “coding tree units.” A coding block is an N×N block of samples. A CU may comprise a coding block of luma samples and two corresponding coding blocks of chroma samples of a picture that has a luma sample array, a Cb sample array, and a Cr sample array, and syntax structures used to code the samples of the coding blocks. In monochrome pictures or pictures having three separate color planes, a CU may comprise a single coding block and syntax structures used to code the samples of the coding block.

Syntax data within a bitstream may also define a size for the CTU. A slice includes a number of consecutive CTUs in coding order. A video frame or picture may be partitioned into one or more slices. As mentioned above, each tree block may be split into coding units (CUs) according to a quad-tree. In general, a quad-tree data structure includes one node per CU, with a root node corresponding to the treeblock. If a CU is split into four sub-CUs, the node corresponding to the CU includes four leaf nodes, each of which corresponds to one of the sub-CUs.

Each node of the quadtree data structure may provide syntax data for the corresponding CU. For example, a node in the quadtree may include a split flag, indicating whether the CU corresponding to the node is split into sub-CUs. Syntax elements for a CU may be defined recursively, and may depend on whether the CU is split into sub-CUs. If a CU is not split further, it is referred as a leaf-CU. If a block of CU is split further, it may be generally referred to as a non-leaf-CU. In some examples of this disclosure, four sub-CUs of a leaf-CU may be referred to as leaf-CUs even if there is no explicit splitting of the original leaf-CU. For example, if a CU at 16×16 size is not split further, the four 8×8 sub-CUs may also be referred to as leaf-CUs although the 16×16 CU was never split.

A CU has a similar purpose as a macroblock of the H.264 standard, except that a CU does not have a size distinction. For example, a tree block may be split into four child nodes (also referred to as sub-CUs), and each child node may in turn be a parent node and be split into another four child nodes. A final, unsplit child node, referred to as a leaf node of the quadtree, comprises a coding node, also referred to as a leaf-CU. Syntax data associated with a coded bitstream may define a maximum number of times a tree block may be split, referred to as a maximum CU depth, and may also define a minimum size of the coding nodes. Accordingly, a bitstream may also define a smallest coding unit (SCU). This disclosure uses the term “block” to refer to any of a CU, PU, or TU, in the context of HEVC, or similar data structures in the context of other standards (e.g., macroblocks and sub-blocks thereof in H.264/AVC).

A CU includes a coding node as well as prediction units (PUs) and transform units (TUs) associated with the coding node. A size of the CU corresponds to a size of the coding node and may be, in some examples, square in shape. In the example of HEVC, the size of the CU may range from 8×8 pixels up to the size of the tree block with a maximum of 64×64 pixels or greater. Each CU may contain one or more PUs and one or more TUs. Syntax data associated with a CU may describe, for example, partitioning of the CU into one or more PUs. Partitioning modes may differ between whether the CU is skip or direct mode encoded, intra-prediction mode encoded, or inter-prediction mode encoded. PUs may be partitioned to be non-square in shape. Syntax data associated with a CU may also describe, for example, partitioning of the CU into one or more TUs according to a quadtree. A TU can be square or non-square (e.g., rectangular) in shape.

The HEVC standard allows for transformations according to TUs. The TUs may be different for different CUs. The TUs are typically sized based on the size of PUs within a given CU defined for a partitioned LCU, although this may not always be the case. The TUs are typically the same size or smaller than the PUs. In some examples, residual samples corresponding to a CU may be subdivided into smaller units using a quad-tree structure, sometimes called a “residual quad tree” (RQT). The leaf nodes of the RQT may be referred to as TUs. Pixel difference values associated with the TUs may be transformed to produce transform coefficients, which may be quantized.

A leaf-CU may include one or more PUs. In general, a PU represents a spatial area corresponding to all or a portion of the corresponding CU, and may include data for retrieving a reference sample for the PU. Moreover, a PU includes data related to prediction. For example, when the PU is intra-mode encoded, data for the PU may be included in a RQT, which may include data describing an intra-prediction mode for a TU corresponding to the PU. As another example, when the PU is inter-mode encoded, the PU may include data defining one or more motion vectors for the PU. The data defining the motion vector for a PU may describe, for example, a horizontal component of the motion vector, a vertical component of the motion vector, a resolution for the motion vector (e.g., one-quarter pixel precision or one-eighth pixel precision), a reference picture to which the motion vector points, and/or a reference picture list (e.g., List 0, List 1, or List C) for the motion vector.

A leaf-CU having one or more PUs may also include one or more TUs. The TUs may be specified using an RQT (also referred to as a TU quad-tree structure), as discussed above. For example, a split flag may indicate whether a leaf-CU is split into four transform units. In some examples, each transform unit may be split further into further sub-TUs. When a TU is not split further, it may be referred to as a leaf-TU. Generally, for intra coding, all the leaf-TUs belonging to a leaf-CU contain residual data produced from the same intra prediction mode. That is, the same intra-prediction mode is generally applied to calculate predicted values that will be transformed in all TUs of a leaf-CU. For intra coding, video encoder 22 may calculate a residual value for each leaf-TU using the intra prediction mode, as a difference between the portion of the CU corresponding to the TU and the original block. A TU is not necessarily limited to the size of a PU. Thus, TUs may be larger or smaller than a PU. For intra coding, a PU may be collocated with a corresponding leaf-TU for the same CU. In some examples, the maximum size of a leaf-TU may correspond to the size of the corresponding leaf-CU.

Moreover, TUs of leaf-CUs may also be associated with respective RQT structures. That is, a leaf-CU may include a quadtree indicating how the leaf-CU is partitioned into TUs. The root node of a TU quadtree generally corresponds to a leaf-CU, while the root node of a CU quadtree generally corresponds to a treeblock (or LCU).

As discussed above, video encoder 22 may partition a coding block of a CU into one or more prediction blocks. A prediction block is a rectangular (i.e., square or non-square) block of samples on which the same prediction is applied. A PU of a CU may comprise a prediction block of luma samples, two corresponding prediction blocks of chroma samples, and syntax structures used to predict the prediction blocks. In monochrome pictures or pictures having three separate color planes, a PU may comprise a single prediction block and syntax structures used to predict the prediction block. Video encoder 22 may generate predictive blocks (e.g., luma, Cb, and Cr predictive blocks) for prediction blocks (e.g., luma, Cb, and Cr prediction blocks) of each PU of the CU.

Video encoder 22 may use intra prediction or inter prediction to generate the predictive blocks for a PU. If video encoder 22 uses intra prediction to generate the predictive blocks of a PU, video encoder 22 may generate the predictive blocks of the PU based on decoded samples of the picture that includes the PU.

After video encoder 22 generates predictive blocks (e.g., luma, Cb, and Cr predictive blocks) for one or more PUs of a CU, video encoder 22 may generate one or more residual blocks for the CU. For instance, video encoder 22 may generate a luma residual block for the CU. Each sample in the CU's luma residual block indicates a difference between a luma sample in one of the CU's predictive luma blocks and a corresponding sample in the CU's original luma coding block. In addition, video encoder 22 may generate a Cb residual block for the CU. Each sample in the Cb residual block of a CU may indicate a difference between a Cb sample in one of the CU's predictive Cb blocks and a corresponding sample in the CU's original Cb coding block. Video encoder 22 may also generate a Cr residual block for the CU. Each sample in the CU's Cr residual block may indicate a difference between a Cr sample in one of the CU's predictive Cr blocks and a corresponding sample in the CU's original Cr coding block.

Furthermore, as discussed above, video encoder 22 may use quad-tree partitioning to decompose the residual blocks (e.g., the luma, Cb, and Cr residual blocks) of a CU into one or more transform blocks (e.g., luma, Cb, and Cr transform blocks). A transform block is a rectangular (e.g., square or non-square) block of samples on which the same transform is applied. A transform unit (TU) of a CU may comprise a transform block of luma samples, two corresponding transform blocks of chroma samples, and syntax structures used to transform the transform block samples. Thus, each TU of a CU may have a luma transform block, a Cb transform block, and a Cr transform block. The luma transform block of the TU may be a sub-block of the CU's luma residual block. The Cb transform block may be a sub-block of the CU's Cb residual block. The Cr transform block may be a sub-block of the CU's Cr residual block. In monochrome pictures or pictures having three separate color planes, a TU may comprise a single transform block and syntax structures used to transform the samples of the transform block.

Video encoder 22 may apply one or more transforms a transform block of a TU to generate a coefficient block for the TU. For instance, video encoder 22 may apply one or more transforms to a luma transform block of a TU to generate a luma coefficient block for the TU. A coefficient block may be a two-dimensional array of transform coefficients. A transform coefficient may be a scalar quantity. Video encoder 22 may apply one or more transforms to a Cb transform block of a TU to generate a Cb coefficient block for the TU. Video encoder 22 may apply one or more transforms to a Cr transform block of a TU to generate a Cr coefficient block for the TU.

In some examples, video encoder 22 skips application of the transforms to the transform block. In such examples, video encoder 22 may treat residual sample values in the same way as transform coefficients. Thus, in examples where video encoder 22 skips application of the transforms, the following discussion of transform coefficients and coefficient blocks may be applicable to transform blocks of residual samples.

After generating a coefficient block (e.g., a luma coefficient block, a Cb coefficient block or a Cr coefficient block), video encoder 22 may quantize the coefficient block to possibly reduce the amount of data used to represent the coefficient block, potentially providing further compression. Quantization generally refers to a process in which a range of values is compressed to a single value. For example, quantization may be done by dividing a value by a constant, and then rounding to the nearest integer. To quantize the coefficient block, video encoder 22 may quantize transform coefficients of the coefficient block. After video encoder 22 quantizes a coefficient block, video encoder 22 may entropy encode syntax elements indicating the quantized transform coefficients. For example, video encoder 22 may perform Context-Adaptive Binary Arithmetic Coding (CABAC) or other entropy coding techniques on the syntax elements indicating the quantized transform coefficients.

Video encoder 22 may output a bitstream that includes a sequence of bits that forms a representation of coded pictures and associated data. Thus, the bitstream comprises an encoded representation of video data. The bitstream may comprise a sequence of network abstraction layer (NAL) units. A NAL unit is a syntax structure containing an indication of the type of data in the NAL unit and bytes containing that data in the form of a raw byte sequence payload (RBSP) interspersed as necessary with emulation prevention bits. Each of the NAL units may include a NAL unit header and may encapsulate a RBSP. The NAL unit header may include a syntax element indicating a NAL unit type code. The NAL unit type code specified by the NAL unit header of a NAL unit indicates the type of the NAL unit. A RBSP may be a syntax structure containing an integer number of bytes that is encapsulated within a NAL unit. In some instances, an RBSP includes zero bits.

Video decoder 30 may receive a bitstream generated by video encoder 22. Video decoder 30 may decode the bitstream to reconstruct pictures of the video data. As part of decoding the bitstream, video decoder 30 may parse the bitstream to obtain syntax elements from the bitstream. Video decoder 30 may reconstruct the pictures of the video data based at least in part on the syntax elements obtained from the bitstream. The process to reconstruct the video data may be generally reciprocal to the process performed by video encoder 22. For instance, video decoder 30 may use motion vectors of PUs to determine predictive blocks for the PUs of a current CU. In addition, video decoder 30 may inverse quantize coefficient blocks of TUs of the current CU. Video decoder 30 may perform inverse transforms on the coefficient blocks to reconstruct transform blocks of the TUs of the current CU. Video decoder 30 may reconstruct the coding blocks of the current CU by adding the samples of the predictive blocks for PUs of the current CU to corresponding samples of the transform blocks of the TUs of the current CU. By reconstructing the coding blocks for each CU of a picture, video decoder 30 may reconstruct the picture.

Common concepts and certain design aspects of HEVC are described below, focusing on techniques for block partition. In HEVC, the largest coding unit in a slice is called a CTB. A CTB is divided according to a quad-tree structure, the nodes of which are coding units. The plurality of nodes in a quad-tree structure includes leaf nodes and non-leaf nodes. The leaf nodes have no child nodes in the tree structure (i.e., the leaf nodes are not further split). The, non-leaf nodes include a root node of the tree structure. The root node corresponds to an initial video block of the video data (e.g., a CTB). For each respective non-root node of the plurality of nodes, the respective non-root node corresponds to a video block that is a sub-block of a video block corresponding to a parent node in the tree structure of the respective non-root node. Each respective non-leaf node of the plurality of non-leaf nodes has one or more child nodes in the tree structure.

The size of a CTB range from 16×16 to 64×64 in the HEVC main profile (although technically 8×8 CTB sizes can be supported). A CTB may be recursively split into CUs in a quad-tree manner, as described in W. J. Han et al, “Improved Video Compression Efficiency Through Flexible Unit Representation and Corresponding Extension of Coding Tools”, IEEE Transaction on Circuits and Systems for Video Technology, vol. 20, no. 12, pp. 1709-1720, December 2010, and shown in FIG. 2. As shown in FIG. 2, each level of partitioning is a quad-tree split into four sub-blocks. The black block is an example of a leaf-node (i.e., a block that is not further split).

In some examples, a CU may be the same size of a CTB, although a CU can be as small as 8×8. Each CU is coded with one coding mode, which could be, e.g., an intra coding mode or an inter coding mode. Other coding modes are also possible, including coding modes for screen content (e.g., intra block copy modes, palette-based coding modes, etc.). When a CU is inter coded (i.e., inter mode is applied), the CU may be further partitioned into prediction units (PUs). For example, a CU may be partitioned in to 2 or 4 PUs. In another example, the entire CU is treated as a single PU when further partitioning is not applied. In HEVC examples, when two PUs are present in one CU, they can be half size rectangles or two rectangle size with ¼ or ¾ size of the CU.

In HEVC, there are eight partition modes for a CU coded with inter prediction mode, i.e., PART_2N×2N, PART_2N×N, PART_N×2N, PART_N×N, PART_2NxnU, PART_2NxnD, PART_nLx2N and PART_nRx2N, as shown in FIG. 3. As shown in FIG. 3, a CU coded with partition mode PART_2N×2N is not further split. That is, the entire CU is treated as a single PU (PU0). A CU coded with partition mode PART_2N×N is symmetrically horizontally split into two PUs (PU0 and PU1). A CU coded with partition mode PART_N×2N is symmetrically vertically split into two PUs. A CU coded with partition mode PART_N×N is symmetrically split into four equal-sized PUs (PU0, PU1, PU2, PU3).

A CU coded with partition mode PART_2NxnU is asymmetrically horizontally split into one PU0 (the upper PU) having ¼ the size of the CU and one PU1 (the lower PU) having ¾ the size of the CU. A CU coded with partition mode PART_2NxnD is asymmetrically horizontally split into one PU0 (the upper PU) having ¾ the size of the CU and one PU1 (the lower PU) having ¼ the size of the CU. A CU coded with partition mode PART_nLx2N is asymmetrically vertically split into one PU0 (the left PU) having ¼ the size of the CU and one PU1 (the right PU) having ¾ the size of the CU. A CU coded with partition mode PART_nRx2N is asymmetrically vertically split into one PU0 (the left PU) having ¾ the size of the CU and one PU1 (the right PU) having ¼ the size of the CU.

When a CU is inter coded, one set of motion information (e.g., motion vector, prediction direction, and reference picture) is present for each PU. In addition, each PU is coded with a unique inter prediction mode to derive the set of motion information. However, it should be understood that even two PUs are coded uniquely, they may still have the same motion information in some circumstances.

In J. An et al., “Block partitioning structure for next generation video coding”, International Telecommunication Union, COM16-C966, September 2015 (hereinafter, “VCEG proposal COM16-C966”), quad-tree-binary-tree (QTBT) partitioning techniques were proposed for future video coding standard beyond HEVC. Simulations have shown that the proposed QTBT structure is more efficient than the quad-tree structure in used HEVC.

In the proposed QTBT structure of VCEG proposal COM16-C966, a CTB is first partitioned using quad-tree portioning techniques, where the quad-tree splitting of one node can be iterated until the node reaches the minimum allowed quad-tree leaf node size. The minimum allowed quad-tree leaf node size may be indicated to video decoder by the value of the syntax element MinQTSize. If the quad-tree leaf node size is not larger than the maximum allowed binary-tree root node size (e.g., as denoted by a syntax element MaxBTSize), the quad-tree leaf node can be further partitioned using binary-tree partitioning. The binary-tree partitioning of one node can be iterated until the node reaches the minimum allowed binary-tree leaf node size (e.g., as denoted by a syntax element MinBTSize) or the maximum allowed binary-tree depth (e.g., as denoted by a syntax element MaxBTDepth). VCEG proposal COM16-C966 uses the term “CU” to refer to binary-tree leaf nodes. In VCEG proposal COM16-C966, CUs are used for prediction (e.g., intra prediction, inter prediction, etc.) and transform without any further partitioning. In general, according to QTBT techniques, there are two splitting types for binary-tree splitting: symmetric horizontal splitting and symmetric vertical splitting. In each case, a block is split by dividing the block down the middle, either horizontally or vertically.

In one example of the QTBT partitioning structure, the CTU size is set as 128×128 (e.g., a 128×128 luma block and two corresponding 64×64 chroma blocks), the MinQTSize is set as 16×16, the MaxBTSize is set as 64×64, the MinBTSize (for both width and height) is set as 4, and the MaxBTDepth is set as 4. The quad-tree partitioning is applied to the CTU first to generate quad-tree leaf nodes. The quad-tree leaf nodes may have a size from 16×16 (i.e., the MinQTSize is 16×16) to 128×128 (i.e., the CTU size). According to one example of QTBT partitioning, if the leaf quad-tree node is 128×128, the leaf quad-tree node cannot be further split by the binary-tree since the size of the leaf quad-tree node exceeds the MaxBTSize (i.e., 64×64). Otherwise, the leaf quad-tree node is further partitioned by the binary-tree. Therefore, the quad-tree leaf node is also the root node for the binary-tree and has the binary-tree depth as 0. The binary-tree depth reaching MaxBTDepth (e.g., 4) implies that there is no further splitting. The binary-tree node having a width equal to the MinBTSize (e.g., 4) implies that there is no further horizontal splitting. Similarly, the binary-tree node having a height equal to MinBTSize implies no further vertical splitting. The leaf nodes of the binary-tree (CUs) are further processed (e.g., by performing a prediction process and a transform process) without any further partitioning.

FIG. 4A illustrates an example of a block 50 (e.g., a CTB) partitioned using QTBT partitioning techniques. As shown in FIG. 4A, using QTBT partition techniques, each of the resultant blocks is split symmetrically through the center of each block. FIG. 4B illustrates the tree structure corresponding to the block partitioning of FIG. 4B. The solid lines in FIG. 4B indicate quad-tree splitting and dotted lines indicate binary-tree splitting. In one example, in each splitting (i.e., non-leaf) node of the binary-tree, a syntax element (e.g., a flag) is signaled to indicate the type of splitting performed (e.g., horizontal or vertical), where 0 indicates horizontal splitting and 1 indicates vertical splitting. For the quad-tree splitting, there is no need to indicate the splitting type, as quad-tree splitting always splits a block horizontally and vertically into 4 sub-blocks with an equal size.

As shown in FIG. 4B, at node 70, block 50 is split into the four blocks 51, 52, 53, and 54, shown in FIG. 4A, using QT partitioning. Block 54 is not further split, and is therefore a leaf node. At node 72, block 51 is further split into two blocks using BT partitioning. As shown in FIG. 4B, node 72 is marked with a 1, indicating vertical splitting. As such, the splitting at node 72 results in block 57 and the block including both blocks 55 and 56. Blocks 55 and 56 are created by a further vertical splitting at node 74. At node 76, block 52 is further split into two blocks 58 and 59 using BT partitioning. As shown in FIG. 4B, node 76 is marked with a 1, indicating horizontal splitting.

At node 78, block 53 is split into 4 equal size blocks using QT partitioning. Blocks 63 and 66 are created from this QT partitioning and are not further split. At node 80, the upper left block is first split using vertical binary-tree splitting resulting in block 60 and a right vertical block. The right vertical block is then split using horizontal binary-tree splitting into blocks 61 and 62. The lower right block created from the quad-tree splitting at node 78, is split at node 84 using horizontal binary-tree splitting into blocks 64 and 65.

In other embodiments, a MTT based CU structure may replace QT, BT, and/or QTBT based CU structures. The MTT partitioning structure is still a recursive tree structure. However, multiple different partition structures (e.g., three or more) are used. For example, according to the MTT techniques of this disclosure, three or more different partition structures may be used at each depth of a tree structure. In this context, the depth of a node in a tree structure may refer to the length of the path (e.g., the number of splits) from the node to the root of the tree structure. As used in this disclosure, a partition structure may generally refer to how many different blocks a block may be divided into. For example, a quad-tree partitioning structure may divide a block into four blocks, a binary-tree partitioning structure may divide a block into two blocks, and a triple-tree partitioning structure may divide a block into three blocks. A partition structure may have multiple different partition types, as will be explained in more detail below. A partition type may additionally define how a block is divided, including symmetric or asymmetric partitioning, uniform or non-uniform partitioning, and/or horizontal or vertical partitioning.

In one example in accordance with the techniques of this disclosure, video encoder 22 may be configured to receive a picture of video data, and partition the picture of video data into a plurality of blocks using three or more different partition structures, and encode the plurality of blocks of the picture of video data. Similarly, video decoder 30 may be configured to receive a bitstream that includes a sequence of bits that forms a representation of a coded picture of video data, determine a partitioning of the coded picture of the video data into a plurality of blocks using three or more different partition structures, and reconstruct the plurality of blocks of the coded picture of the video data. In one example, partitioning the frame of video data comprises partitioning the frame of video data into the plurality of blocks using the three or more different partition structures, wherein at least three of the three or more different partition structures may be used at each depth of a tree structure that represents how the frame of video data is partitioned. In one example, the three or more different partition structures include a triple-tree partition structure, and video encoder 22 and/or video decoder 30 may be configured to partition one of the plurality of blocks of video data using a triple-tree partition type of the triple-tree partition structure, wherein the triple-tree partition structure divides the one of the plurality of blocks of into three sub-blocks without dividing the one of the plurality of blocks through the center. In a further example of the disclosure, the three or more different partition structures further include a quad-tree partition structure and a binary-tree partition structure.

Thus, in one example, video encoder 22 may generate an encoded representation of an initial video block (e.g., a coding tree block or CTU) of video data. As part of generating the encoded representation of the initial video block, video encoder 22 determines a tree structure comprising a plurality of nodes. For example, video encoder 22 may partition a tree block using the MTT partitioning structure of this disclosure.

The plurality of nodes in the MTT partitioning structure includes a plurality of leaf nodes and a plurality of non-leaf nodes. The leaf nodes have no child nodes in the tree structure. The non-leaf nodes include a root node of the tree structure. The root node corresponds to the initial video block. For each respective non-root node of the plurality of nodes, the respective non-root node corresponds to a video block (e.g., a coding block) that is a sub-block of a video block corresponding to a parent node in the tree structure of the respective non-root node. Each respective non-leaf node of the plurality of non-leaf nodes has one or more child nodes in the tree structure. In some examples, a non-leaf node at a picture boundary may only have one child node due to a forced split and one of the child nodes corresponds to a block outside the picture boundary.

According to the techniques of this disclosure, for each respective non-leaf node of the tree structure at each depth level of the tree structure, there is a plurality of allowed splitting patterns (e.g., partition structure) for the respective non-leaf node. For example, there may be three or more partition structures allowed for each depth of the tree structure. Video encoder 22 may be configured to partition a video block corresponding to the respective non-leaf node into video blocks corresponding to the child nodes of the respective non-leaf node according to one of the plurality of allowable partition structure. Each respective allowed partition structure of the plurality of allowed partition structures may correspond to a different way of partitioning the video block corresponding to the respective non-leaf node into video blocks corresponding to the child nodes of the respective non-leaf node. Furthermore, in this example, video encoder 22 may include the encoded representation of the initial video block in a bitstream that comprises an encoded representation of the video data.

In a similar example, video decoder 30 may determine a tree structure comprising a plurality of nodes. As in the previous example, the plurality of nodes includes a plurality of leaf nodes and a plurality of non-leaf nodes. The leaf nodes have no child nodes in the tree structure. The non-leaf nodes include a root node of the tree structure. The root node corresponds to an initial video block of the video data. For each respective non-root node of the plurality of nodes, the respective non-root node corresponds to a video block that is a sub-block of a video block corresponding to a parent node in the tree structure of the respective non-root node. Each respective non-leaf node of the plurality of non-leaf nodes has one or more child nodes in the tree structure. For each respective non-leaf node of the tree structure at each depth level of the tree structure, there are a plurality of allowed splitting patterns for the respective non-leaf node and the video block corresponding to the respective non-leaf node is partitioned into video blocks corresponding to the child nodes of the respective non-leaf node according to one of the plurality of allowable splitting patterns. Each respective allowed splitting pattern of the plurality of allowed splitting patterns corresponds to a different way of partitioning the video block corresponding to the respective non-leaf node into video blocks corresponding to the child nodes of the respective non-leaf node. Furthermore, in this example, for each (or at least one) respective leaf node of the tree structure, video decoder 30 reconstructs the video block corresponding to the respective leaf node.

In some such examples, for each respective non-leaf node of the tree structure other than the root node, the plurality of allowed splitting patterns (e.g., partition structures) for the respective non-leaf node is independent of the partition structure according to which a video block corresponding to a parent node of the respective non-leaf node is partitioned into video blocks corresponding to child nodes of the parent node of the respective non-leaf node.

In other examples of the disclosure, at each depth of the tree structure, video encoder 22 may be configured to further split sub-trees using a particular partition type from among one of three more partitioning structures. For example, video encoder 22 may be configured to determine a particular partition type from QT, BT, triple-tree (TT) and other partitioning structures. In one example, the QT partitioning structure may include square quad-tree and rectangular quad-tree partitioning types. Video encoder 22 may partition a square block using square quad-tree partitioning by dividing the block, down the center both horizontally and vertically, into four equal-sized square blocks. Likewise, video encoder 22 may partition a rectangular (e.g., non-square) block using rectangular quad-tree partition by dividing the rectangular block, down the center both horizontally and vertically, into four equal-sized rectangular blocs.

The BT partitioning structure may include horizontal symmetric binary-tree, vertical symmetric binary-tree, horizontal non-symmetric binary-tree, and vertical non-symmetric binary-tree partition types. For the horizontal symmetric binary-tree partition type, video encoder 22 may be configured to split a block, down the center of the block horizontally, into two symmetric blocks of the same size. For the vertical symmetric binary-tree partition type, video encoder 22 may be configured to split a block, down the center of the block vertically, into two symmetric blocks of the same size. For the horizontal non-symmetric binary-tree partition type, video encoder 22 may be configured to split a block, horizontally, into two blocks of differing size. For example, one block may be ¼ the size of the parent block and the other block may be ¾ the size of the parent blocks, as in the PART_2NxnU or PART_2NxnD partition type of FIG. 3. For the vertical non-symmetric binary-tree partition type, video encoder 22 may be configured to split a block, vertically, into two blocks of differing size. For example, one block may be ¼ the size of the parent block and the other block may be ¾ the size of the parent blocks, as in the PART_nLx2N or PART_nRx2N partition type of FIG. 3.

In other examples, an asymmetric binary-tree partition type may divide a parent block into different size fractions. For example, one sub-block may be ⅜ of the parent block and the other sub-block may be ⅝ of the parent block. Of course, such a partition type may be either vertical or horizontal.

The TT partition structure differs from that of the QT or BT structures, in that the TT partition structure does not split a block down the center. The center region of the block remains together in the same sub-block. Different from QT, which produces four blocks, or binary tree, which produces two blocks, splitting according to a TT partition structure produces three blocks. Example partition types according to the TT partition structure include symmetric partition types (both horizontal and vertical), as well as asymmetric partition types (both horizontal and vertical). Furthermore, the symmetric partition types according to the TT partition structure may be uneven/non-uniform or even/uniform. The asymmetric partition types according to the TT partition structure of this disclosure are uneven/non-uniform. In one example of the disclosure, a TT partition structure may include the following partition types: horizontal even/uniform symmetric triple-tree, vertical even/uniform symmetric triple-tree, horizontal uneven/non-uniform symmetric triple-tree, vertical uneven/non-uniform symmetric triple-tree, horizontal uneven/non-uniform asymmetric triple-tree, and vertical uneven/non-uniform asymmetric triple-tree partition types.

In general, an uneven/non-uniform symmetric triple-tree partition type is a partition type that is symmetric about a center line of the block, but where at least one of the resultant blocks three blocks is not the same size as the other two. One preferred example is where the side blocks are ¼ the size of the block, and the center block is ½ the size of the block. An even/uniform symmetric triple-tree partition type is a partition type that is symmetric about a center line of the block, and the resultant blocks are all the same size. Such a partition is possible if the block height or width, depending on a vertical or horizontal split, is a multiple of 3. An uneven/non-uniform asymmetric triple-tree partition type is a partition type that is not symmetric about a center line of the block, and where at least one of the resultant blocks is not the same size as the other two.

FIG. 5A is a conceptual diagram illustrating example horizontal triple-tree partition types. FIG. 5B is a conceptual diagram illustrating example vertical triple-tree partition types. In both FIG. 5A and FIG. 5B, h represents the height of the block in luma or chroma samples and w represents the width of the block in luma or chroma samples. Note that the respective “center lines” in each of the triple-tree partitions in FIGS. 5A and 5B do not represent the boundary of the block (i.e., the triple-tree partitions do not spit a block through the center line). Rather, the center lines are shown to depict whether or not a particular partition type is symmetric or asymmetric relative to the center line of the original block. The depicted center lines are also along the direction of the split.

As shown in FIG. 5A, block 71 is partitioned with a horizontal even/uniform symmetric partition type. The horizontal even/uniform symmetric partition type produces symmetrical top and bottom halves relative to the center line of block 71. The horizontal even/uniform symmetric partition type produces three sub-blocks of equal size, each with a height of h/3 and a width of w. The horizontal even/uniform symmetric partition type is possible when the height of block 71 is evenly divisible by 3.

Block 73 is partitioned with a horizontal uneven/non-uniform symmetric partition type. The horizontal uneven/non-uniform symmetric partition type produces symmetrical top and bottom halves relative to the center line of block 73. The horizontal uneven/non-uniform symmetric partition type produces two blocks of equal size (e.g., the top and bottom blocks with a height of h/4), and a center block of a different size (e.g., a center block with a height of h/2). In one example of the disclosure, according to the horizontal uneven/non-uniform symmetric partition type, the area of the center block is equal to the combined areas of the top and bottom blocks. In some examples, the horizontal uneven/non-uniform symmetric partition type may be preferred for blocks having a height that is a power of 2 (e.g., 2, 4, 8, 16, 32, etc.).

Block 75 is partitioned with a horizontal uneven/non-uniform asymmetric partition type. The horizontal uneven/non-uniform asymmetric partition type does not produce a symmetrical top and bottom half relative to the center line of block 75 (i.e., the top and bottom halves are asymmetric). In the example of FIG. 5A, the horizontal uneven/non-uniform asymmetric partition type produces a top block with height of h/4, a center block with height of 3h/8, and a bottom block with a height of 3h/8. Of course, other asymmetric arrangements may be used.

As shown in FIG. 5B, block 77 is partitioned with a vertical even/uniform symmetric partition type. The vertical even/uniform symmetric partition type produces symmetrical left and right halves relative to the center line of block 77. The vertical even/uniform symmetric partition type produces three sub-blocks of equal size, each with a width of w/3 and a height of h. The vertical even/uniform symmetric partition type is possible when the width of block 77 is evenly divisible by 3.

Block 79 is partitioned with a vertical uneven/non-uniform symmetric partition type. The vertical uneven/non-uniform symmetric partition type produces symmetrical left and right halves relative to the center line of block 79. The vertical uneven/non-uniform symmetric partition type produces symmetrical left and right halves relative to the center line of 79. The vertical uneven/non-uniform symmetric partition type produces two blocks of equal size (e.g., the left and right blocks with a width of w/4), and a center block of a different size (e.g., a center block with a width of w/2). In one example of the disclosure, according to the vertical uneven/non-uniform symmetric partition type, the area of the center block is equal to the combined areas of the left and right blocks. In some examples, the vertical uneven/non-uniform symmetric partition type may be preferred for blocks having a width that is a power of 2 (e.g., 2, 4, 8, 16, 32, etc.).

Block 81 is partitioned with a vertical uneven/non-uniform asymmetric partition type. The vertical uneven/non-uniform asymmetric partition type does not produce a symmetrical left and right half relative to the center line of block 81 (i.e., the left and right halves are asymmetric). In the example of FIG. 5B, the vertical uneven/non-uniform asymmetric partition type produces a left block with width of w/4, a center block with width of 3w/8, and a bottom block with a width of 3w/8. Of course, other asymmetric arrangements may be used.

In examples where a block (e.g., at a sub-tree node) is split to a non-symmetric triple-tree partition type, video encoder 22 and/or video decoder 30 may apply a restriction such that two of the three partitions have the same size. Such a restriction may correspond to a limitation to which video encoder 22 must comply when encoding video data. Furthermore, in some examples, video encoder 22 and video decoder 30 may apply a restriction whereby the sum of the area of two partitions is equal to the area of the remaining partition when splitting according to a non-symmetric triple-tree partition type. For instance, video encoder 22 may generate or video decoder 30 may receive an encoded representation of the initial video block that complies with a restriction specifying that when a video block corresponding to a node of the tree structure is partitioned according to a non-symmetric triple-tree pattern, the node has a first child node, a second child node, and a third child node, the second child node corresponding to a video block between video blocks corresponding to the first and third child nodes, the video blocks corresponding to the first and third child nodes have the same size, and a sum of the sizes of the video blocks corresponding to the first and third child nodes is equal to a size of the video block corresponding to the second child node.

FIG. 5C is a conceptual diagram illustrating example asymmetric binary tree partition types. Embodiments may include an asymmetric binary tree (ABT) CU coding tool on top of the QTBT structure. In ABT structure, the set of binary tree split modes of QTBT is extended to support asymmetric binary splitting of a coding unit. To be specific, a CU can be divided into 2 sub-CUs with respective size ¼ and ¾ the size of the parent CU. 4 new Binary Tree splitting modes have been introduced into the QTBT framework, so as to allow new splitting configurations. So-called asymmetric splitting modes may be used in addition to the splitting modes already available in QTBT, as shown in FIG. 5C. Further details of ABT structures can be found in F. Le Leannec, T. Poirier, F. Urban, “Asymmetric Coding Units in QTBT”, International Telecommunication Union, JVET-D0064, October 2016.

In some examples of the disclosure, video encoder 22 may be configured to select from among all the of the aforementioned partition types for each of the QT, BT, ABT, and TT partition structures. In other examples, video encoder 22 may be configured to only determine a partition type from among a subset of the aforementioned partition types. For example, a subset of the above-discussed partition types (or other partition types) may be used for certain block sizes or for certain depths of a quadtree structure. The subset of supported partition types may be signaled in the bitstream for use by video decoder 30 or may be predefined such that video encoder 22 and video decoder 30 may determine the subsets without any signaling.

In other examples, the number of supported partitioning types may be fixed for all depths in all CTUs. That is, video encoder 22 and video decoder 30 may be preconfigured to use the same number of partitioning types for any depth of a CTU. In other examples, the number of supported partitioning types may vary and may be dependent on depth, slice type, or other previously coded information. In one example, at depth 0 or depth 1 of the tree structure, only the QT partition structure is used. At depths greater than 1, each of the QT, BT, and TT partition structures may be used.

In some examples, video encoder 22 and/or video decoder 30 may apply preconfigured constraints on supported partitioning types in order to avoid duplicated partitioning for a certain region of a video picture or region of a CTU. In one example, when a block is split with non-symmetric partition type, video encoder 22 and/or video decoder 30 may be configured to not further split the largest sub-block that is split from the current block. For example, when a square block is split according to a non-symmetric partition type (e.g., PART_2NxnU partition type in FIG. 3), the largest sub-block among all sub-blocks (e.g., PU1 of PART_2NxnU partition type in FIG. 3) is the noted leaf node and cannot be further split. However, the smaller sub-block (e.g., PU0 of PART_2NxnU partition type in FIG. 3) can be further split.

As another example where constraints on supported partitioning types may be applied to avoid duplicated partitioning for a certain region, when a block is split with non-symmetric partition type, the largest sub-block that is split from the current block cannot be further split in the same direction. For example, when a square block is split non-symmetric partition type (e.g., PART_2NxnU partition type in FIG. 3), video encoder 22 and/or video decoder 30 may be configured to not split the large sub-block among all sub-blocks (e.g. PU1 of PART_2NxnU partition type in FIG. 3) in the horizontal direction. However, video encoder 22 and/or video decoder 30, in this example, may split PU1 again in the vertical direction.

As another example where constraints on supported partitioning types may be applied to avoid difficulty in further splitting, video encoder 22 and/or video decoder 30 may be configured to not split a block, either horizontally or vertically, when the width/height of a block is not a power of 2 (e.g., when the width height is not 2, 4, 8, 16, etc.).

The above examples describe how video encoder 22 may be configured to perform MTT partitioning according techniques of this disclosure. Video decoder 30 may also then apply the same MTT partitioning as was performed by video encoder 22. In some examples, how a picture of video data was partitioned by video encoder 22 may be determined by applying the same set of predefined rules at video decoder 30. However, in many situations, video encoder 22 may determine a particular partition structure and partition type to use based on rate-distortion criteria for the particular picture of video data being coded. As such, in order for video decoder 30 to determine the partitioning for a particular picture, video encoder 22 may signal syntax elements in the encoded bitstream that indicate how the picture, and CTUs of the picture, are to be partitioned. Video decoder 30 may parse such syntax elements and partition the picture and CTUs accordingly.

In one example of the disclosure, video encoder 22 may be configured to signal a particular subset of supported partition types as a high-level syntax element, in a sequence parameter set (SPS), a picture parameter set (PPS), slice header, adaptive parameter set (APS), or any other high level syntax parameter set. For example, the maximum number of partition types and which types are supported may be predefined, or signaled in bitstream as a high-level syntax element, in sequence parameter set (SPS), picture parameter set (PPS) or any other high level syntax parameter sets. Video decoder 30 may be configured to receive and parse such a syntax element to determine the particular subset of partition types that are use and/or the maximum number of partition structures (e.g., QT, BT, TT, etc.) and types that are supported.

In some examples, at each depth, video encoder 22 may be configured to signal an index that indicates the selected partitioning type used at that depth of the tree structure. Furthermore, in some examples, video encoder 22 may adaptively signal such a partition type index at each CU, i.e., the index could be different for different CUs. For instance, video encoder 22 may set the index of the partitioning type based on one or more rate-distortion calculations. In one example, the signaling of the partitioning type (e.g., the index of the partitioning type) may be skipped if certain condition is satisfied. For example, video encoder 22 may skip signaling of the partition type when there is only one supported partitioning type associated with a specific depth. In this example, when nearing a picture boundary, a region to be coded may be smaller than a CTU. Consequently, in this example, CTUs may be forced to be split to fit for the picture boundary. In one example, only symmetric binary-tree is used for the forced split and no partitioning type is signaled. In some examples, at a certain depth, the partitioning type may be derived based on previously coded information, such as slice type, CTU depth, CU position.

In another example of the disclosure, for each CU (leaf node), video encoder 22 may be further configured to signal a syntax element (e.g., a one-bit transform_split flag) to indicate whether a transform is to be performed on the same size of the CU or not (i.e., the flag indicates whether the TU is the same size of the CU or is further split). In case the transform_split flag is signaled as true, video encoder 22 may be configured to further split the residual of the CU into multiple sub-blocks and the transform is done on each sub-block. Video decoder 30 may perform the reciprocal process.

In one example, when the transform_split flag is signaled as true, the following is performed. If the CU corresponds to a square block (i.e., the CU is square), then video encoder 22 splits the residual into four square sub-blocks using quad-tree splitting, and the transform is performed on each square sub-block. If the CU corresponds to a non-square block, e.g., M×N, then video encoder 22 splits the residual into two sub-blocks, and the sub-block size is 0.5M×N when M>N, and M×0.5N when M<N. As another example, when the transform_split flag is signaled as true and the CU corresponds to a non-square block, e.g., M×N, (i.e., the CU is non-square), video encoder 22 may be configured to split the residual into sub-blocks with size K×K, and the K×K square transform is used for each sub-block, where K is equal to the maximal factor of M and N. As another example, no transform_split flag is signaled when a CU is a square block.

In some examples, no split flag is signaled and only a transform with one derived size is used for when there is residue in the CU after prediction. For example, a CU with size equal to M×N, the K×K square transform is used, where K is equal to the maximal factor of M and N. Thus, in this example, for a CU with size 16×8, the same 8×8 transform may be applied to two 8×8 sub-blocks of residual data of the CU. A “split flag” is a syntax element indicating that a node in a tree structure has child nodes in the tree structure.

In some examples, for each CU, if the CU is not split to a squared quad-tree, or a symmetric binary-tree, video encoder 22 is configured to always set the transform size equal to the size of the partition (e.g., the size the CU).

Simulation results have shown that, compared to JEM-3.1 reference software, the coding performance using the MTT techniques of disclosure, in the case of random access, has shown in improvement. On average, simulations have shown that the MTT techniques of this disclosure have provided a 3.18% bitrate-distortion (BD)-rate reduction with only a moderate encoding time increase. Simulations have shown that the MTT techniques of this disclosure provide good performance for higher resolutions, e.g., 4.20% and 4.89% luma BD-rate reduction for Class A1 and Class A2 testing. Class A1 and Class A2 are example 4K resolution test sequences.

It should be understood that, for each of the above examples described with reference to video encoder 22, video decoder 30 may be configured to perform a reciprocal process. With regard to signaling syntax elements, video decoder 30 may be configured to receive and parse such syntax element and partition and decode the associated video data accordingly.

In one specific example of the disclosure, video decoder may be configured to partition video blocks according to three different partition structures (QT, BT, and TT), with five different partitioning types allowed at each depth. The partitioning types include quad-tree partitioning (QT partition structure), horizontal binary-tree partitioning (BT partition structure), vertical binary-tree partitioning (BT partition structure), horizontal center-side triple-tree partitioning (TT partition structure), and vertical center-side tripe-tree partitioning (TT partition structure), as shown in FIGS. 5A-5E.

The definitions of the five partitioning types are as follows. Please note that square is regarded as a special case of rectangular.

-   -   Quad-tree partitioning: a block is further split into four         same-size rectangular blocks. FIG. 6A shows an example of         quad-tree partitioning.     -   Vertical binary-tree partitioning: a block is vertically split         into two same-size rectangular blocks. FIG. 6B is an example of         vertical binary-tree partitioning.     -   Horizontal binary-tree partitioning: a block is horizontally         split into two same-size rectangular blocks. FIG. 6C is an         example of horizontal binary-tree partitioning.     -   Vertical center-side triple-tree partitioning: a block is         vertically split into three rectangular blocks so that the two         side blocks share the same size while the size of the center         block is the sum of the two side blocks. FIG. 6D is an example         of vertical center-side triple-tree partitioning.     -   Horizontal center-side triple-tree partitioning: a block is         horizontally split into three rectangular blocks so that the two         side blocks share the same size while the size of the center         block is the sum of the two side blocks. FIG. 6E is an example         of horizontal center-side triple-tree partitioning.

For a block associated with a particular depth, video encoder 22 determines which partitioning type (including no further split) is used and signals the determined partition type explicitly or implicitly (e.g., the partition type may be derived from predetermined rules) to video decoder 30. Video encoder 22 may determine the partition type to use based on checking rate-distortion cost for the block using different partition types. To get the rate distortion cost, video encoder 22 may need to check possible partitioning types for the block, recursively.

FIG. 7 is a conceptual diagram illustrating an example of coding tree unit (CTU) partitioning. In other words, FIG. 7 illustrates the partitioning of a CTB 91 corresponding to a CTU. In the example of FIG. 7,

-   -   At depth 0, CTB 91 (i.e., the whole CTB) is split into two         blocks with horizontal binary-tree partitioning (as indicated by         line 93 with dashes separated by single dots).     -   At depth 1:         -   The upper block is split into three blocks with vertical             center-side triple-tree partitioning (as indicated by lines             95 and 86 with small dashes).         -   The bottom block is split into four blocks with quad-tree             partitioning (as indicated by lines 88 and 90 with dashes             separated by two dots).     -   At depth 2:         -   The left side block of the upper block at depth 1 is split             into three blocks with horizontal center-side triple-tree             partitioning (as indicated by lines 92 and 94 with long             dashes separated by short dashes).         -   No further split for the center and right blocks of the             upper block at depth 1.         -   No further split for the four blocks of the bottom block at             depth 1.

As can be seen in the example of FIG. 7, three different partition structures are used (BT, QT, and TT) with four different partition types (horizontal binary-tree partitioning, vertical center-side triple-tree partitioning, quad-tree partitioning, and horizontal center-side triple-tree partitioning).

In another example, additional constraints may be applied for block at a certain depth or with a certain size. For example, with the height/width of a block is smaller than 16 pixels, the block cannot be split with vertical/horizontal center-side tree to avoid a block with height/width smaller than 4 pixels.

Although QTBT, ABT and TT structures discussed herein may have better coding performance than the quad-tree structure used in HEVC, further flexibility is desirable. For example, in some examples, a CU leaf node in a BT, ABT, TT structure can be further split into recursive transform unit.

The RQT in HEVC only allows splitting a residual block of a CU into TUs in quad-tree structure, which not flexible enough. In some examples, a CU's residual block can be split into TUs flexibly in QT, BT, ABT, TT or other structures recursively.

Transform Unit Partitioning Structure

To achieve more flexible partitioning for a CTU, in some embodiments, a residual block of a CU can be further split into transform units recursively in the way of QT, BT, ABT, TT or any other partition or any combination of them. For example, the video encoder 22 or decoder 30 may determine a partitioning of a coded into a plurality of coded units according to a first tree structure that includes a leaf node. The video encoder 22 or decoder 30 may determine that a residual block of the leaf node is recursively split into a plurality of transform units according to a second tree structure.

In one example, a flag is coded in a video bitstream (encoded into the bitstream via a video encoder and decoded from the bitstream via a video decoder) at the root of the transform unit tree such that only a single one transform unit partition structure for this second tree is allowed in the corresponding tree for entire residual block. For example, as one example, if only vertical center-side tree is allowed in this tree, a TU in this tree can be split into vertical center-side trees only.

In another example, a flag is coded for each partitioned TU or group (e.g., subset) of TUs within the same block to determine in which structure the TU will be split.

Transform Information Signal

Optionally, for a TU, a transform tool such as enhanced multiple transform (EMT), non-separable secondary transform (NSST) and transform skip (TS) can be applied. In one example, a flag of a transform tool is only coded (encoded by the encodesr in a bitstream and decoded from such a bitstream by a decoder) at the root of a transform unit tree, such that all TUs in the tree share the same flag. For example, in one such example, an EMT flag and index are signaled only at the root level. This EMT flag and index will determine whether EMT is applied and which set of EMT is applied to all the TUs in the same tree. In another example, a flag of a transform tool is coded for each TU or group of TUs. For example, the EMT flag and index are coded for each TU. Each TU will perform EMT based on its own flag.

Coefficient Coding

In HEVC, coded block flag (CBF) and coefficients are coded for each TU. This coding may suffer from extensive overhead by signalling CBFs and last position of non-zero coefficient. According to some embodiments, in order to reduce this overhead, coefficients of the whole transform unit tree (in other words for all TUs) can be signaled together. In such examples, only CBFs and last position of non-zero coefficient at the root level is signaled.

Various examples have been described. Particular examples of this disclosure may be used separately or in combination with one another.

FIG. 8 is a block diagram illustrating an example video encoder 22 that may implement the techniques of this disclosure. FIG. 8 is provided for purposes of explanation and should not be considered limiting of the techniques as broadly exemplified and described in this disclosure. The techniques of this disclosure may be applicable to various coding standards or methods.

In the example of FIG. 8, video encoder 22 includes a prediction processing unit 100, video data memory 101, a residual generation unit 102, a transform processing unit 104, a quantization unit 106, an inverse quantization unit 108, an inverse transform processing unit 110, a reconstruction unit 112, a filter unit 114, a decoded picture buffer 116, and an entropy encoding unit 118. Prediction processing unit 100 includes an inter-prediction processing unit 120 and an intra-prediction processing unit 126. Inter-prediction processing unit 120 may include a motion estimation unit and a motion compensation unit (not shown).

Video data memory 101 may be configured to store video data to be encoded by the components of video encoder 22. The video data stored in video data memory 101 may be obtained, for example, from video source 18. Decoded picture buffer 116 may be a reference picture memory that stores reference video data for use in encoding video data by video encoder 22, e.g., in intra- or inter-coding modes. Video data memory 101 and decoded picture buffer 116 may be formed by any of a variety of memory devices, such as dynamic random access memory (DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM), resistive RAM (RRAM), or other types of memory devices. Video data memory 101 and decoded picture buffer 116 may be provided by the same memory device or separate memory devices. In various examples, video data memory 101 may be on-chip with other components of video encoder 22, or off-chip relative to those components. Video data memory 101 may be the same as or part of storage media 20 of FIG. 1.

Video encoder 22 receives video data. Video encoder 22 may encode each CTU in a slice of a picture of the video data. Each of the CTUs may be associated with equally-sized luma coding tree blocks (CTBs) and corresponding CTBs of the picture. As part of encoding a CTU, prediction processing unit 100 may perform partitioning to divide the CTBs of the CTU into progressively-smaller blocks. The smaller blocks may be coding blocks of CUs. For example, prediction processing unit 100 may partition a CTB associated with a CTU according to a tree structure. In accordance with one or more techniques of this disclosure, for each respective non-leaf node of the tree structure at each depth level of the tree structure, there are a plurality of allowed splitting patterns for the respective non-leaf node and the video block corresponding to the respective non-leaf node is partitioned into video blocks corresponding to the child nodes of the respective non-leaf node according to one of the plurality of allowable splitting patterns. In one example, prediction processing unit 100 or another processing unit of video encoder 22 may be configured to perform any combination of the MTT partitioning techniques described above.

Video encoder 22 may encode CUs of a CTU to generate encoded representations of the CUs (i.e., coded CUs). As part of encoding a CU, prediction processing unit 100 may partition the coding blocks associated with the CU among one or more PUs of the CU. In accordance with techniques of this disclosure, a CU may only include a single PU. That is, in some examples of this disclosure, a CU is not divided into separate prediction blocks, but rather, a prediction process is performed on the entire CU. Thus, each CU may be associated with a luma prediction block and corresponding chroma prediction blocks. Video encoder 22 and video decoder 30 may support CUs having various sizes. As indicated above, the size of a CU may refer to the size of the luma coding block of the CU also the size of a luma prediction block. As discussed above, video encoder 22 and video decoder 30 may support CU sizes defined by any combination of the example MTT partitioning types described above.

Inter-prediction processing unit 120 may generate predictive data for a PU by performing inter prediction on each PU of a CU. As explained above, in some MTT examples of this disclosure a CU may contain only a single PU, that is, the CU and PU may be synonymous. The predictive data for the PU may include predictive blocks of the PU and motion information for the PU. Inter-prediction processing unit 120 may perform different operations for a PU or a CU depending on whether the PU is in an I slice, a P slice, or a B slice. In an I slice, all PUs are intra predicted. Hence, if the PU is in an I slice, inter-prediction processing unit 120 does not perform inter prediction on the PU. Thus, for blocks encoded in I-mode, the predicted block is formed using spatial prediction from previously-encoded neighboring blocks within the same picture. If a PU is in a P slice, inter-prediction processing unit 120 may use uni-directional inter prediction to generate a predictive block of the PU. If a PU is in a B slice, inter-prediction processing unit 120 may use uni-directional or bi-directional inter prediction to generate a predictive block of the PU.

Intra-prediction processing unit 126 may generate predictive data for a PU by performing intra prediction on the PU. The predictive data for the PU may include predictive blocks of the PU and various syntax elements. Intra-prediction processing unit 126 may perform intra prediction on PUs in I slices, P slices, and B slices.

To perform intra prediction on a PU, intra-prediction processing unit 126 may use multiple intra prediction modes to generate multiple sets of predictive data for the PU. Intra-prediction processing unit 126 may use samples from sample blocks of neighboring PUs to generate a predictive block for a PU. The neighboring PUs may be above, above and to the right, above and to the left, or to the left of the PU, assuming a left-to-right, top-to-bottom encoding order for PUs, CUs, and CTUs. Intra-prediction processing unit 126 may use various numbers of intra prediction modes, e.g., 33 directional intra prediction modes. In some examples, the number of intra prediction modes may depend on the size of the region associated with the PU.

Prediction processing unit 100 may select the predictive data for PUs of a CU from among the predictive data generated by inter-prediction processing unit 120 for the PUs or the predictive data generated by intra-prediction processing unit 126 for the PUs. In some examples, prediction processing unit 100 selects the predictive data for the PUs of the CU based on rate/distortion metrics of the sets of predictive data. The predictive blocks of the selected predictive data may be referred to herein as the selected predictive blocks.

Residual generation unit 102 may generate, based on the coding blocks (e.g., luma, Cb and Cr coding blocks) for a CU and the selected predictive blocks (e.g., predictive luma, Cb and Cr blocks) for the PUs of the CU, residual blocks (e.g., luma, Cb and Cr residual blocks) for the CU. For instance, residual generation unit 102 may generate the residual blocks of the CU such that each sample in the residual blocks has a value equal to a difference between a sample in a coding block of the CU and a corresponding sample in a corresponding selected predictive block of a PU of the CU.

Transform processing unit 104 may perform quad-tree partitioning to partition the residual blocks associated with a CU into transform blocks associated with TUs of the CU. Thus, a TU may be associated with a luma transform block and two chroma transform blocks. The sizes and positions of the luma and chroma transform blocks of TUs of a CU may or may not be based on the sizes and positions of prediction blocks of the PUs of the CU. A quad-tree structure known as a “residual quad-tree” (RQT) may include nodes associated with each of the regions. The TUs of a CU may correspond to leaf nodes of the RQT. In other examples, transform processing unit 104 may be configured to partition TUs in accordance with the MTT techniques described above. For example, video encoder 22 may not further divide CUs into TUs using an RQT structure. As such, in one example, a CU includes a single TU.

Transform processing unit 104 may generate transform coefficient blocks for each TU of a CU by applying one or more transforms to the transform blocks of the TU. Transform processing unit 104 may apply various transforms to a transform block associated with a TU. For example, transform processing unit 104 may apply a discrete cosine transform (DCT), a directional transform, or a conceptually similar transform to a transform block. In some examples, transform processing unit 104 does not apply transforms to a transform block. In such examples, the transform block may be treated as a transform coefficient block.

Quantization unit 106 may quantize the transform coefficients in a coefficient block. The quantization process may reduce the bit depth associated with some or all of the transform coefficients. For example, an n-bit transform coefficient may be rounded down to an m-bit transform coefficient during quantization, where n is greater than m. Quantization unit 106 may quantize a coefficient block associated with a TU of a CU based on a quantization parameter (QP) value associated with the CU. Video encoder 22 may adjust the degree of quantization applied to the coefficient blocks associated with a CU by adjusting the QP value associated with the CU. Quantization may introduce loss of information. Thus, quantized transform coefficients may have lower precision than the original ones.

Inverse quantization unit 108 and inverse transform processing unit 110 may apply inverse quantization and inverse transforms to a coefficient block, respectively, to reconstruct a residual block from the coefficient block. Reconstruction unit 112 may add the reconstructed residual block to corresponding samples from one or more predictive blocks generated by prediction processing unit 100 to produce a reconstructed transform block associated with a TU. By reconstructing transform blocks for each TU of a CU in this way, video encoder 22 may reconstruct the coding blocks of the CU.

Filter unit 114 may perform one or more deblocking operations to reduce blocking artifacts in the coding blocks associated with a CU. Decoded picture buffer 116 may store the reconstructed coding blocks after filter unit 114 performs the one or more deblocking operations on the reconstructed coding blocks. Inter-prediction processing unit 120 may use a reference picture that contains the reconstructed coding blocks to perform inter prediction on PUs of other pictures. In addition, intra-prediction processing unit 126 may use reconstructed coding blocks in decoded picture buffer 116 to perform intra prediction on other PUs in the same picture as the CU.

Entropy encoding unit 118 may receive data from other functional components of video encoder 22. For example, entropy encoding unit 118 may receive coefficient blocks from quantization unit 106 and may receive syntax elements from prediction processing unit 100. Entropy encoding unit 118 may perform one or more entropy encoding operations on the data to generate entropy-encoded data. For example, entropy encoding unit 118 may perform a CABAC operation, a context-adaptive variable length coding (CAVLC) operation, a variable-to-variable (V2V) length coding operation, a syntax-based context-adaptive binary arithmetic coding (SBAC) operation, a Probability Interval Partitioning Entropy (PIPE) coding operation, an Exponential-Golomb encoding operation, or another type of entropy encoding operation on the data. Video encoder 22 may output a bitstream that includes entropy-encoded data generated by entropy encoding unit 118. For instance, the bitstream may include data that represents the partition structure for a CU according to the techniques of this disclosure.

FIG. 9 is a block diagram illustrating an example video decoder 30 that is configured to implement the techniques of this disclosure. FIG. 9 is provided for purposes of explanation and is not limiting on the techniques as broadly exemplified and described in this disclosure. For purposes of explanation, this disclosure describes video decoder 30 in the context of HEVC coding. However, the techniques of this disclosure may be applicable to other coding standards or methods.

In the example of FIG. 9, video decoder 30 includes an entropy decoding unit 150, video data memory 151, a prediction processing unit 152, an inverse quantization unit 154, an inverse transform processing unit 156, a reconstruction unit 158, a filter unit 160, and a decoded picture buffer 162. Prediction processing unit 152 includes a motion compensation unit 164 and an intra-prediction processing unit 166. In other examples, video decoder 30 may include more, fewer, or different functional components.

Video data memory 151 may store encoded video data, such as an encoded video bitstream, to be decoded by the components of video decoder 30. The video data stored in video data memory 151 may be obtained, for example, from computer-readable medium 16, e.g., from a local video source, such as a camera, via wired or wireless network communication of video data, or by accessing physical data storage media. Video data memory 151 may form a coded picture buffer (CPB) that stores encoded video data from an encoded video bitstream. Decoded picture buffer 162 may be a reference picture memory that stores reference video data for use in decoding video data by video decoder 30, e.g., in intra- or inter-coding modes, or for output. Video data memory 151 and decoded picture buffer 162 may be formed by any of a variety of memory devices, such as dynamic random access memory (DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM), resistive RAM (RRAM), or other types of memory devices. Video data memory 151 and decoded picture buffer 162 may be provided by the same memory device or separate memory devices. In various examples, video data memory 151 may be on-chip with other components of video decoder 30, or off-chip relative to those components. Video data memory 151 may be the same as or part of storage media 28 of FIG. 1.

Video data memory 151 receives and stores encoded video data (e.g., NAL units) of a bitstream. Entropy decoding unit 150 may receive encoded video data (e.g., NAL units) from video data memory 151 and may parse the NAL units to obtain syntax elements. Entropy decoding unit 150 may entropy decode entropy-encoded syntax elements in the NAL units. Prediction processing unit 152, inverse quantization unit 154, inverse transform processing unit 156, reconstruction unit 158, and filter unit 160 may generate decoded video data based on the syntax elements extracted from the bitstream. Entropy decoding unit 150 may perform a process generally reciprocal to that of entropy encoding unit 118.

In accordance with some examples of this disclosure, entropy decoding unit 150, or another processing unit of video decoder 30, may determine a tree structure as part of obtaining the syntax elements from the bitstream. The tree structure may specify how an initial video block, such as a CTB, is partitioned into smaller video blocks, such as coding units. In accordance with one or more techniques of this disclosure, for each respective non-leaf node of the tree structure at each depth level of the tree structure, there are a plurality of allowed partition types for the respective non-leaf node and the video block corresponding to the respective non-leaf node is partitioned into video blocks corresponding to the child nodes of the respective non-leaf node according to one of the plurality of allowable splitting patterns. In other embodiments, residual blocks corresponding to CU leaf-nodes may be split according to a determined tree structure.

In addition to obtaining syntax elements from the bitstream, video decoder 30 may perform a reconstruction operation on a non-partitioned CU. To perform the reconstruction operation on a CU, video decoder 30 may perform a reconstruction operation on each TU of the CU. By performing the reconstruction operation for each TU of the CU, video decoder 30 may reconstruct residual blocks of the CU. As discussed above, in one example of the disclosure, a CU includes a single TU.

As part of performing a reconstruction operation on a TU of a CU, inverse quantization unit 154 may inverse quantize, i.e., de-quantize, coefficient blocks associated with the TU. After inverse quantization unit 154 inverse quantizes a coefficient block, inverse transform processing unit 156 may apply one or more inverse transforms to the coefficient block in order to generate a residual block associated with the TU. For example, inverse transform processing unit 156 may apply an inverse DCT, an inverse integer transform, an inverse Karhunen-Loeve transform (KLT), an inverse rotational transform, an inverse directional transform, or another inverse transform to the coefficient block.

If a CU or PU is encoded using intra prediction, intra-prediction processing unit 166 may perform intra prediction to generate predictive blocks of the PU. Intra-prediction processing unit 166 may use an intra prediction mode to generate the predictive blocks of the PU based on samples spatially-neighboring blocks. Intra-prediction processing unit 166 may determine the intra prediction mode for the PU based on one or more syntax elements obtained from the bitstream.

If a PU is encoded using inter prediction, entropy decoding unit 150 may determine motion information for the PU. Motion compensation unit 164 may determine, based on the motion information of the PU, one or more reference blocks. Motion compensation unit 164 may generate, based on the one or more reference blocks, predictive blocks (e.g., predictive luma, Cb and Cr blocks) for the PU. As discussed above, in one example of the disclosure using MTT partitioning, a CU may include only a single PU. That is, a CU may not be divided into multiple PUs.

Reconstruction unit 158 may use transform blocks (e.g., luma, Cb and Cr transform blocks) for TUs of a CU and the predictive blocks (e.g., luma, Cb and Cr blocks) of the PUs of the CU, i.e., either intra-prediction data or inter-prediction data, as applicable, to reconstruct the coding blocks (e.g., luma, Cb and Cr coding blocks) for the CU. For example, reconstruction unit 158 may add samples of the transform blocks (e.g., luma, Cb and Cr transform blocks) to corresponding samples of the predictive blocks (e.g., luma, Cb and Cr predictive blocks) to reconstruct the coding blocks (e.g., luma, Cb and Cr coding blocks) of the CU.

Filter unit 160 may perform a deblocking operation to reduce blocking artifacts associated with the coding blocks of the CU. Video decoder 30 may store the coding blocks of the CU in decoded picture buffer 162. Decoded picture buffer 162 may provide reference pictures for subsequent motion compensation, intra prediction, and presentation on a display device, such as display device 32 of FIG. 1. For instance, video decoder 30 may perform, based on the blocks in decoded picture buffer 162, intra prediction or inter prediction operations for PUs of other CUs.

FIG. 10A is a flowchart illustrating an example operation of video encoder 22, in accordance with a technique of this disclosure. In the example of FIG. 10A, video encoder 22 may generate an encoded representation of an initial video block (e.g., a coding tree block) of video data (200). As part of generating the encoded representation of the initial video block, video encoder 22 determines a tree structure comprising a plurality of nodes. The plurality of nodes includes a plurality of leaf nodes and a plurality of non-leaf nodes. The leaf nodes have no child nodes in the tree structure. The non-leaf nodes include a root node of the tree structure. The root node corresponds to the initial video block. For each respective non-root node of the plurality of nodes, the respective non-root node corresponds to a video block (e.g., a coding block) that is a sub-block of a video block corresponding to a parent node in the tree structure of the respective non-root node. Each respective non-leaf node of the plurality of non-leaf nodes has one or more child nodes in the tree structure. For each respective non-leaf node of the tree structure at each depth level of the tree structure, there are a plurality of allowed partition types of three or more partition structures (e.g., BT, QT, and TT partition structures) for the respective non-leaf node and the video block corresponding to the respective non-leaf node is partitioned into video blocks corresponding to the child nodes of the respective non-leaf node according to one of the plurality of allowable partition types. Each respective allowed partition types of the plurality of allowed partition types may correspond to a different way of partitioning the video block corresponding to the respective non-leaf node into video blocks corresponding to the child nodes of the respective non-leaf node. Furthermore, in this example, video encoder 22 may include the encoded representation of the initial video block in a bitstream that comprises an encoded representation of the video data (202).

FIG. 10B is a flowchart illustrating an example operation of video decoder 30, in accordance with a technique of this disclosure. In the example of FIG. 10B, video decoder 30 may determine a tree structure comprising a plurality of nodes (250). The plurality of nodes includes a plurality of leaf nodes and a plurality of non-leaf nodes the leaf nodes have no child nodes in the tree structure. The non-leaf nodes include a root node of the tree structure. The root node corresponds to an initial video block of the video data. For each respective non-root node of the plurality of nodes, the respective non-root node corresponds to a video block that is a sub-block of a video block corresponding to a parent node in the tree structure of the respective non-root node. Each respective non-leaf node of the plurality of non-leaf nodes has one or more child nodes in the tree structure. For each respective non-leaf node of the tree structure at each depth level of the tree structure, there are a plurality of allowed partition types of three or more partition structures (e.g., BT, QT, and TT partition structures) for the respective non-leaf node and the video block corresponding to the respective non-leaf node is partitioned into video blocks corresponding to the child nodes of the respective non-leaf node according to one of the plurality of allowable partition types. Each respective allowed partition type of the plurality of allowed partition types corresponds to a different way of partitioning the video block corresponding to the respective non-leaf node into video blocks corresponding to the child nodes of the respective non-leaf node. Furthermore, in this example, for each (or at least one) respective leaf node of the tree structure, video decoder 30 reconstructs the video block corresponding to the respective leaf node (252).

In the examples of FIG. 10A and FIG. 10B, for each respective non-leaf node of the tree structure other than the root node, the plurality of allowed partition types for the respective non-leaf node may be independent of a splitting pattern according to which a video block corresponding to a parent node of the respective non-leaf node is partitioned into video blocks corresponding to child nodes of the parent node of the respective non-leaf node. For instance, unlike VCEG proposal COM16-C966, if the video block of a particular node is split according to a binary-tree splitting pattern, the video block of a child node of the particular node may be split according to a quad-tree splitting pattern.

Furthermore, in the examples of FIG. 10A and FIG. 10B, for each respective non-leaf node of the tree structure, the plurality of allowed splitting patterns for the respective non-leaf node may include two or more of: a squared quad-tree splitting pattern, a rectangular quad-tree splitting pattern, a symmetric binary-tree splitting pattern, a non-symmetric binary-tree splitting pattern, a symmetric triple-tree splitting pattern, or a non-symmetric triple-tree splitting pattern.

Furthermore, as indicated above, only a subset of afore-mentioned partition types is used. The subset of supported partition types may be signaled in the bitstream or predefined. Thus, in some examples, video decoder 30 may obtain, from a bitstream, syntax elements indicating a plurality of supported splitting patterns. Similarly, video encoder 22 may signal, in the bitstream, a plurality of supported splitting patterns. In these examples, for each respective non-leaf node of the tree structure, the plurality of supported splitting patterns may include the plurality of allowed splitting patterns for the respective non-leaf node. In these examples, the syntax elements indicating the plurality of supported splitting patterns may be obtained from (and signaled in) a bitstream, such as in a sequence parameter set (SPS) or a picture parameter set (PPS), or slice header.

As indicated above, in some examples, when the sub-tree is split to a non-symmetric triple-tree, a restriction is applied that the two of the three partitions have the same size. Accordingly, in some examples, video decoder 30 may receive an encoded representation of the initial video block that complies with a restriction specifying that when a video block corresponding to a node of the tree structure is partitioned according to a non-symmetric triple-tree pattern, video blocks corresponding to two child nodes of the node have the same size. Likewise, video encoder 22 may generate the encoded representation of the initial video block to comply with a restriction specifying that when a video block corresponding to a node of the tree structure is partitioned according to a non-symmetric triple-tree pattern, video blocks corresponding to two child nodes of the node have the same size.

As indicated above, in some examples, the number of supported partitioning types may be fixed for all depths in all CTUs. For instance, there may be the same number of allowed splitting patterns in the plurality of allowed splitting patterns for each non-leaf node of the tree structure. Additionally, as indicated above, in other examples, the number of supported partitioning types may be dependent on depth, slice type, CTU type or other previously coded information. For instance, for at least one non-leaf node of the tree structure, the number of allowed splitting patterns in the plurality of allowed splitting patterns for the non-leaf node is dependent on at least one of: a depth of the non-leaf node in the tree structure, the size of the video block corresponding to the non-leaf node in the tree structure, a slice type or previously-coded information.

In some examples, when a block is split with a non-symmetric partition type (e.g., the non-symmetric binary-tree partition types shown in FIG. 3, including PART_2NxnU, PART_2NxnD, PART_nLx2N, PART_nRx2N), the largest sub-block that is split from the current block cannot be further split. For instance, a constraint on how the initial video block is encoded may require that, when a video block corresponding to any non-leaf node of the tree structure is split into a plurality of sub-blocks according to a non-symmetric splitting pattern, the largest sub-block of the plurality of sub-blocks corresponds to a leaf node of the tree structure.

In some examples, when a block is split with non-symmetric partition type, the largest sub-block that is split from the current block cannot be further split in the same direction. For instance, a constraint on how the initial video block is encoded may require that, when a video block corresponding to any non-leaf node of the tree structure is split in a first direction into a plurality of sub-blocks according to a non-symmetric splitting pattern, the tree structure cannot contain nodes corresponding to sub-blocks of the largest sub-block of the plurality of sub-blocks that are split from the largest sub-block of the plurality of sub-block in the first direction.

In some examples, when the width/height of a block is not a power of 2, no further horizontal/vertical split is allowed. For instance, a constraint on how the initial video block is encoded may require a node of the tree structure corresponding to a video block whose height or width is not a power of 2 to be a leaf node.

In some examples, at each depth, an index of the selected partitioning type is signaled in bitstream. Thus, in some examples, video encoder 22 may include, in the bitstream, an index of a splitting pattern according to which a video block corresponding to a non-leaf node of the tree structure is split into video blocks corresponding to child nodes of the non-leaf node. Similarly, in some examples, video decoder 30 may obtain, from a bitstream, an index of a splitting pattern according to which a video block corresponding to a non-leaf node of the tree structure is split into video blocks corresponding to child nodes of the non-leaf node.

In some examples, for each CU (leaf node), a 1-bit transform_split flag is further signaled to indicate whether the transform is done with same size of the CU or not. In case the transform_split flag is signaled as true, the residual of the CU is further split into multiple sub-blocks and transform is done on each sub-block. Accordingly, in one example, for at least one leaf node of the tree structure, video encoder 22 may include a syntax element in the bitstream. In this example, the syntax element having a first value indicates a transform having the same size as the video block corresponding to the leaf node is applied to residual data of the video block corresponding to the leaf node; the syntax element having a second value indicates multiple transforms having smaller sizes than the video block corresponding to the leaf node are applied to sub-blocks of the residual data of the video block corresponding to the leaf node. In a similar example, for at least one leaf node of the tree structure, video decoder 30 may obtain this syntax element from a bitstream.

In some examples, no split flag is signaled and only transform with one derived size is used for when there is residue in the CU. For instance, for at least one leaf node of the tree structure, video encoder 22 may apply a same transform (e.g., a discrete cosine transform, discrete sine transform, etc.) to different parts of residual data corresponding to the video block corresponding to the leaf node to convert the residual data from a sample domain to a transform domain. In the sample domain, the residual data is represented in terms of values of samples (e.g., components of pixels). In the transform domain, the residual data may be represented in terms of frequency coefficients. Likewise, for at least one leaf node of the tree structure, video decoder 30 may apply a same transform (i.e., an inverse discrete cosine transform, inverse sine transform, etc.) to different parts of residual data corresponding to the video block corresponding to the leaf node to convert the residual data from a transform domain to a sample domain.

In some examples, for each CU, if the CU is not split to a squared quad-tree, or a symmetric binary-tree, the transform size is always set equal to the size of the partition size. For instance, for each respective non-leaf node of the tree structure corresponding to a video block partitioned according to a squared quad-tree splitting pattern or symmetric binary-tree splitting pattern, transform sizes of transforms applied to residual data of video blocks corresponding to child nodes of the respective non-leaf node are always set equal to the sizes of the video blocks corresponding to the child nodes of the respective non-leaf node.

FIG. 11 is a flowchart illustrating an example operation of a video encoder, in accordance with another example technique of this disclosure. One or more structural elements of video encoder 22, including prediction processing unit 100, may be configured to perform the techniques of FIG. 11.

In one example of the disclosure, video encoder 22 may be configured to obtain a picture of the video data. At a block 300, the video encoder 22 determines a partitioning of the coded picture of the video data into a plurality of coded units according to a first tree structure that includes a leaf node. The first tree structure may be determined using a rate-distortion metric based search. Next at a block 302, the encoder 22 determines that a residual block of the leaf node is recursively split into a plurality of transform units according to a second tree structure. The determination to split the residual block may also be based on a search using a rate-distortion metric. At a block 304, the video encoder 22 encodes the picture based on the first and second tree structures. Not shown, the video encoder 22 also encodes each of the other coded units using similar techniques.

FIG. 12 is a flowchart illustrating an example operation of a video decoder, in accordance with another example technique of this disclosure. One or more structural elements of video decoder 30, including entropy decoding unit 150 and/or prediction processing unit 152, may be configured to perform the techniques of FIG. 12.

In one example of the disclosure, video decoder 30 is configured to receive an encoded video bitstream that forms a representation of a coded picture of the video data. At a block 400, the video decoder 30 determines a partitioning of the coded picture of the video data into a plurality of coded units according to a first tree structure that includes a leaf node. At a block 402, the video decoder determines that a residual block of the leaf node is recursively split into a plurality of transform units according to a second tree structure. Moving to a block 404, the video decoder 30 reconstructs the coded picture based on the determined first and second tree structures. Similar techniques can be applied by the video decoder 30 to reconstruct each of the coded units of the picture.

Certain aspects of this disclosure have been described with respect to extensions of the HEVC standard for purposes of illustration. However, the techniques described in this disclosure may be useful for other video coding processes, including other standard or proprietary video coding processes not yet developed, including VVC.

A video coder, as described in this disclosure, may refer to a video encoder or a video decoder. Similarly, a video coding unit may refer to a video encoder or a video decoder. Likewise, video coding may refer to video encoding or video decoding, as applicable. In this disclosure, the phrase “based on” may indicate based only on, based at least in part on, or based in some way on. This disclosure may use the term “video unit” or “video block” or “block” to refer to one or more sample blocks and syntax structures used to code samples of the one or more blocks of samples. Example types of video units may include CTUs, CUs, PUs, transform units (TUs), macroblocks, macroblock partitions, and so on. In some contexts, discussion of PUs may be interchanged with discussion of macroblocks or macroblock partitions. Example types of video blocks may include coding tree blocks, coding blocks, and other types of blocks of video data.

It is to be recognized that depending on the example, certain acts or events of any of the techniques described herein can be performed in a different sequence, may be added, merged, or left out altogether (e.g., not all described acts or events are necessary for the practice of the techniques). Moreover, in certain examples, acts or events may be performed concurrently, e.g., through multi-threaded processing, interrupt processing, or multiple processors, rather than sequentially.

In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media, or communication media including any medium that facilitates transfer of a computer program from one place to another, e.g., according to a communication protocol. In this manner, computer-readable media generally may correspond to (1) tangible computer-readable storage media which is non-transitory or (2) a communication medium such as a signal or carrier wave. Data storage media may be any available media that can be accessed by one or more computers or one or more processors to retrieve instructions, code and/or data structures for implementation of the techniques described in this disclosure. A computer program product may include a computer-readable medium.

By way of example, and not limitation, such computer-readable storage media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage, or other magnetic storage devices, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. For example, if instructions are transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. It should be understood, however, that computer-readable storage media and data storage media do not include connections, carrier waves, signals, or other transitory media, but are instead directed to non-transitory, tangible storage media. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc, where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media.

Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. In addition, in some aspects, the functionality described herein may be provided within dedicated hardware and/or software modules configured for encoding and decoding, or incorporated in a combined codec. Also, the techniques could be fully implemented in one or more circuits or logic elements.

The techniques of this disclosure may be implemented in a wide variety of devices or apparatuses, including a wireless handset, an integrated circuit (IC) or a set of ICs (e.g., a chip set). Various components, modules, or units are described in this disclosure to emphasize functional aspects of devices configured to perform the disclosed techniques, but do not necessarily require realization by different hardware units. Rather, as described above, various units may be combined in a codec hardware unit or provided by a collection of interoperative hardware units, including one or more processors as described above, in conjunction with suitable software and/or firmware.

Various examples have been described. These and other examples are within the scope of the following claims. 

What is claimed is:
 1. A method of decoding video data, the method comprising: receiving an encoded video bitstream that forms a representation of a coded picture of the video data; determining a partitioning of the coded picture of the video data into a plurality of coded units, the partitioning being according to a first tree structure and the plurality of coded units including a leaf node of the first tree structure; determining that a residual block of the leaf node is recursively split into a plurality of transform units according to a second tree structure; and reconstructing the coded picture based on the determined first and second tree structures.
 2. The method of claim 1, wherein the residual block is recursively split into the second tree structure according to at least one of a quad-tree, a binary tree, an asymmetric binary tree, or a triple tree.
 3. The method of claim 1, further comprising decoding, from the encoded video bitstream, a syntax element indicating that only a single transform unit partition structure is allowed in the tree for the residual block.
 4. The method of claim 1, further comprising decoding, from the encoded video bitstream, a syntax element for each of the plurality of transform units indicative of the structure into which the corresponding transform unit is split.
 5. The method of claim 1, further comprising decoding, from the encoded video bitstream, a syntax element for a subset of the plurality of transform units indicative of the structure into which the corresponding group of the transform unit is split.
 6. The method of claim 1, further comprising decoding, from the encoded video bitstream, a syntax element associated with a root of a transform unit tree indicating that the plurality of transform units in the tree are transformed according to a specified transform.
 7. The method of claim 6, wherein the syntax element indicates that the plurality of TUs share parameters associated with one of an enhanced multiple transform (EMT), non-separable secondary transform (NSST) or a transform skip (TS).
 8. The method of claim 1, further comprising decoding, from the encoded video bitstream, a syntax element associated with a root of a transform unit tree indicating at least one of a coded block flag or last position of a non-zero coefficient for the transform unit tree.
 9. A method of encoding video data, the method comprising: obtaining a picture for encoding into an encoded video bitstream; determining a partitioning of the picture of the video data into a plurality of coded units, the partitioning being according to a first tree structure and the plurality of coded units including a leaf node of the first tree structure; determining that a residual block of the leaf node is recursively split into a plurality of transform units according to a second tree structure; and encoding, into the encoded video bitstream, the picture based on the determined first and second tree structures.
 10. The method of claim 9, wherein the residual block is recursively split into the second tree structure according to at least one of a quad-tree, a binary tree, an asymmetric binary tree, or a triple tree.
 11. The method of claim 9, further comprising encoding a syntax element into the encoded video bitstream indicating that only a single transform unit partition structure is allowed in the tree for the residual block.
 12. The method of claim 9, further comprising encoding a syntax element into the encoded video bitstream for each of the plurality of transform units indicative of the structure into which the corresponding transform unit is split.
 13. The method of claim 9, further comprising encoding a syntax element into the encoded video bitstream for a subset of the plurality of transform units indicative of the structure into which the corresponding group of the transform unit is split.
 14. The method of claim 9, further comprising encoding a syntax element into the encoded video bitstream associated with a root of a transform unit tree indicating that the plurality of transform units in the tree are transformed according to a specified transform.
 15. The method of claim 14, wherein the syntax element indicates that the plurality of TUs share parameters associated with one of an enhanced multiple transform (EMT), non-separable secondary transform (NSST) or a transform skip (TS).
 16. The method of claim 9, further comprising encoding a syntax element into the encoded video bitstream associated with a root of a transform unit tree indicating at least one of a coded block flag or last position of a non-zero coefficient for the transform unit tree.
 17. An apparatus for decoding video data, the apparatus comprising: a memory configured to store a coded picture of the video data; and a video processor configured to: receive an encoded video bitstream that forms a representation of the coded picture of the video data; determine a partitioning of the coded picture of the video data into a plurality of coded units, the partitioning being according to a first tree structure and the plurality of coded units including a leaf node of the first tree structure; determine that a residual block of the leaf node is recursively split into a plurality of transform units according to a second tree structure; and reconstruct the coded picture based on the determined first and second tree structures.
 18. The apparatus of claim 17, wherein the video processor is further configured to decode, from the encoded video bitstream, a syntax element indicating that only a single transform unit partition structure is allowed in the tree for the residual block.
 19. The apparatus of claim 17, wherein the video processor is further configured to decode, from the encoded video bitstream, a syntax element for each of the plurality of transform units indicative of the structure into which the corresponding transform unit is split.
 20. The apparatus of claim 17, wherein the video processor is further configured to decode, from the encoded video bitstream, a syntax element indicating that only a single transform unit partition structure is allowed in the tree for the residual block. 